A hydraulic foot-operated service brake system installed on a vehicle is equipped with a power-assisting device such as a servo or booster so that only a relatively light pedal force is required to brake the vehicle. Compressed air, negative pressure in the intake manifold (negative pressure booster), or fluid pressure (hydraulic booster) is used as the power source.
A hydraulic booster utilizing fluid pressure makes use of hydraulic pressure supplied by a hydraulic pressure source to augment the force applied to a hydraulic brake control device such as a master cylinder in response to movement of the brake pedal. A hydraulic booster of this construction is described, for example, in Japanese Patent Laid-Open No. 205545/1986. This known hydraulic booster comprises a housing connected with a master cylinder, an input piston, a power piston sliding in a booster chamber formed inside the housing, and a reaction piston inserted in a hole formed in the power piston. The power piston responds to the input rod and transmits boosted force to the master cylinder. The reaction piston can move a given distance relative to the input rod in response to the power piston. A reaction chamber communicating with the booster chamber is formed by the reaction piston within said hole in the power piston.
In the above-described conventional booster, both power piston and reaction piston are made of metal, and their relative movement is restricted by bearing against each other. Therefore, when the brake pedal is pushed down suddenly, or when the brake pedal force is so strong that the capability of the servo is surpassed, a clunk is produced because both pistons bear against each other. Hence, the driver feels uncomfortable in pushing down on the brake pedal.